1. Field of the Invention
The present invention relates to a digital light-processing projection apparatus and a beam splitter module used in conjunction with the projection apparatus, and especially relates to the design of the digital light-processing projection apparatus and a beam splitter element of the beam splitter module.
2. Descriptions of the Related Art
FIG. 1 is a schematic view of a digital light-processing projection apparatus disclosed in ROC (Taiwan) Patent Application No. 093101928, which was filed by the assignee of the subject application on 29 Jan. 2004. The digital light-processing projection apparatus comprises an optical combiner module 200, a light source 300, a beam splitter module 400, a plurality of Digital Micro-mirror Devices (DMDs) 500R, 500G, 500B, and a projection lens 600.
A light beam W is emitted from the light source 300, passes through a rod integrator 310 and enters the beam splitter module 400, which in turn, splits the light beam W into three primary colour lights R, G, B. Dichroic mirrors 402, 404 split colour lights R, G and colour light B, respectively. Colour lights R, G, B then enter the optical combiner module 200 by respective condenser lenses 406, 408, 410 and reflection mirrors 412, 414, 416, 418, 420 and 422. Colour light B is reflected onto incident plane 242a, while colour light G is reflected onto incident plane 232a. Colour light R, G, B are reflected onto the respective DMDs 500R, 500G, 500B by respective air gaps in Total Internal Reflection (TIR) prisms 220a, 230a, 240a, from which they are subsequently reflected and pass through the respective TIR prisms 220a, 230a, 240a. The projection lens 600 is disposed in the light paths of the respective colour lights R, G, B after the optical combiner module 200. Back focal length 550 is the back focal length of the digital light-processing projection apparatus.
As shown in FIG. 2A, which illustrates a spectrum curve diagram of the colour light R in the beam splitter module 400. Curve S1 represents the spectrum of S-polarized light when the incident angle of colour light R is 45 degrees. Curve P1 represents the spectrum of P-polarized light when the incident angle of colour light R is 45 degrees. Curve S2 represents the spectrum of S-polarized light when the incident angle of colour light R is 52 degrees. Curve P2 represents the spectrum of P-polarized light when the incident angle of colour light R is 52 degrees.
Referring to FIG. 2B, which illustrates a spectrum curve diagram of the colour light R in the optical combiner module 200. Curve S3 represents the spectrum of S-polarized light when the incident angle of colour light R is 45 degrees. Curve P3 represents the spectrum of P-polarized light when the incident angle of colour light R is 45 degrees. Curve S4 represents the spectrum of S-polarized light when the incident angle of colour light R is 52 degrees. Curve P4 represents the spectrum of P-polarized light when the incident angle of colour light R is 52 degrees.
Now referring to FIGS. 2A and 2B, as shown in curves S1, P1, S3 and P3, when the wave length of S-polarized light of colour light R is greater than 570 nm, the S-polarized light of colour light R starts to reflect, while the P-polarized light of colour light R does not reflect until the wave length thereof is greater than 600 nm and the rate of reflection is close to 100%. In other words, the S-polarized light is more suitable for reflection regardless of light splitting or combining.
After two reflections of fold mirrors, the S-polarized light of red light is converted to the P-polarized light. As for curve S1, when the S-polarized light of colour light R in the beam splitter module 400 enters the optical combiner module 200, the curve spectrum becomes curve P3.
Given above, when the wave length of the S-polarized light of colour light R in the beam splitter module 400 is between 570 nm and 600 nm, the light will be transmitted in the optical combiner module 200, which in turn, results in light loss. The same also applies to colour lights B and G. If light loss occurs, the intensity of images by the projection lens is reduced, which in turn, affects the quality and effects of projected images.